1. Field of the Invention
This invention relates to improvements in a vibration wave motor for driving a moving member by a travelling vibration wave.
2. Description of the Prior Art
A vibration wave motor has a number of electro-mechanical energy converting elements, such as electrostrictive elements, adhesively disposed on a vibratory member formed of a resilient material along the direction of travel of a moving member, such that a flexural travelling wave is generated in the vibratory member. A drive force resulting from the vibration wave motion, The frictionally driven moving member placed on the vibratory member An example of such motor according to the prior art is shown in FIGS. 1 and 2 of the accompanying drawings.
In FIGS. 1 and 2, reference numeral 1 designates a vibratory member of a resilient material forming an elliptical closed loop comprising two straight portions and two arcuate portions. Reference numeral 2 denotes a number of electrostrictive elements joined to the vibratory member 1. The electrostrictive elements 2 are disposed in A phase and B phase on the two straight portions of the vibratory member 1 and with a phase difference of 1/4 wavelength (.lambda./4) between the two phases A and B, and are joined to the underside of the vibratory member 1 so that in the respective phases, the adjacent elements are arranged at an interval of (.lambda./2) and are alternately opposite in polarity such as + and -. Reference numeral 3 designates a moving member in frictional contact with the upper surface of the vibratory member 1, and reference numeral 4 denotes a vibration preventing support member for the vibratory member 1.
Alternating voltages of the same frequency having a time phase difference of 90.degree. therebetween are applied to the electrostrictive elements 2 in the A phase and the B phase. Thereupon, two standing waves of flexural vibration by the electrostrictive elements in the A phase and the B phase are generated in the vibratory member 1, but since as described above, the A phase and B phase spatially deviate from each other by (.lambda./4) and have a time phase difference of 90.degree. therebetween, a travelling wave of flexural vibration is generated in the vibratory member 1 as a result of the combination of these standing waves. In this travelling wave of flexural vibration, the points on the neutral surface in the vertical thickness of the vibratory member (the intermediate surface in the vertical thickness) only vibrate in a vertical direction, while the points on the upper and lower surfaces of the vibratory member effect a kind of elliptical motion comprising a combination of vertical and circumferential vibrations with respect to the vibratory member. Accordingly, the moving member in contact with the upper surface of the vibratory member 1 is driven for movement along the vibratory member 1 by the friction force in the contact portion thereof.
A vibration wave motor using a circular vibratory member instead of an elliptical vibratory member is also known and the principle of the driving thereof is similar to what has been described above.
In an elliptical type vibration wave motor, the vibratory member has straight portions and arcuate portions, and the vibration mode when a standing wave has been generated is complicated as compared with a simply circular or straight beam. This is due to the fact that the arrangement of the electrostrictive elements is not symmetrical with respect to the minor axis of the ellipse and that because generally in the vibration wave motor, the electrostrictive elements 2 are joined to the vibratory member, the rigidity of the vibratory member in the joined portion thereof differs from that of the other portions and the vibration mode is further complicated as compared with the case of a single-piece vibratory member. For this reason, as previously described, standing waves have a spatial phase difference of (.lambda./4) in the A phase and the B phase, but deviation between wavelengths occurs due to the above-mentioned factor and the travelling wave comprising these two standing waves superposed one upon the other becomes non-uniform in amplitude, and this unavoidably leads to a disadvantage in the performance of the motor.